1. Field of the Invention
The invention pertains to enhancement of flow mixing by the use of flow-induced cavity resonance. More particularly, it pertains to passive control of mixing in high speed, compressible shear flows by a frequency tunable cavity.
2. Description of the Prior Art
Rapid and effective mixing in a fluid flow or jet is important in many applications such as mixing fuel and air for combustion in a turbulent shear layer between faster and slower moving fluid. Mixing in such a layer can be controlled by acoustic forcing at a frequency close to a naturally occurring frequency of vortical structures in the layer. In subsonic flow at low to intermediate Mach numbers the shear layer is relatively incompressible and has vortical structures that are relatively large and coherent in that their energy is contained in a particular frequency band or bands so that, in the prior art, the structures were excited by low level forcing using an active actuators such as a loudspeaker or the rotating valve shown in United States Statutory Invention Registration (SIR) H1007 which is incorporated in the present application by reference.
However, with higher Mach number flows at realistic Reynolds number conditions, as in supersonic combustion engines, the shear layer is compressible and the naturally occurring structures are small and are not very coherent so that high amplitude forcing is necessary to excite vortical structures effective for mixing. As a result, such prior art actuators are not powerful enough to overcome inherent disturbances from turbulence in the shear layer.
The forcing frequency or frequencies necessary to excite vortical structures, which are effective for mixing or other purposes, vary with factors such as the speed of the fluid, its material and temperature, and size of an associated nozzle so that, to be practically useful, an arrangement for excitation of such structures must be tunable to excite any frequency in a wide range of such frequencies. For enhanced mixing in some applications, it is desirable that such an arrangement provide excitation at more than one frequency. It is highly desirable that such forcing at the high amplitudes required by high speed flow be provided in such a way that energy of the flow not be substantially reduced or its direction undesirably deflected. It is also desirable that arrangements providing such forcing be adaptable to flow from nozzles of different geometries.